Method and apparatus for MV scaling with increased effective scaling ratio

ABSTRACT

A method and apparatus for deriving a scaled MV (motion vector) for a current block based on a candidate MV associated with a candidate block are disclosed. Embodiments according to the present invention increase effective scaling factor of motion vector scaling. In one embodiment, a distance ratio of a first picture distance between a current picture and a target reference picture pointed to by a current motion vector of the current block to a second picture distance between a candidate picture corresponding to the candidate block and a candidate reference picture pointed to by the candidate MV is computed. The scaled MV is then generated based on the candidate MV according to the distance ratio, where the scaled MV has an effective scaling ratio between −m and n, and wherein m and n are positive integers greater than 4. The values of m and n can be 8, 16 or 32.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to U.S. Provisional PatentApplication, Ser. No. 61/556,480, filed on Nov. 7, 2011, entitled“Division-Free MV Scaling”. The U.S. Provisional Patent Application ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to video coding. In particular, thepresent invention relates to derivation of motion vector predictor (MVP)by scaling a candidate motion vector with increased effective scalingratio for High Efficiency Video Coding (HEVC).

BACKGROUND

High-Efficiency Video Coding (HEVC) is a new international video codingstandard that is being developed by the Joint Collaborative Team onVideo Coding (JCT-VC). HEVC is based on the hybrid block-basedmotion-compensated DCT-like transform coding architecture. The basicunit for compression, termed Coding Unit (CU), is a 2N×2N square block,and each CU can be recursively split into four smaller CUs until apredefined minimum size is reached. Each CU contains one or multiplePrediction Units (PUs). The PU sizes can be 2N×2N, 2N×N, 2N×nU, 2N×nD,N×2N, nL×2N, nR×2N, or N×N, where 2N×N, 2N×nU, 2N×nD and N×2N, nL×2N,nR×2N correspond to horizontal and vertical partition of a 2N×2N PU withsymmetric or asymmetric PU size division respectively.

To further increase the coding efficiency of motion vector coding inHEVC, the motion vector competition (MVC) based scheme is applied toselect one motion vector predictor (MVP) among a given MVP candidate setwhich includes spatial and temporal MVPs. There are threeinter-prediction modes including Inter, Skip, and Merge in the HEVC testmodel version 3.0 (HM-3.0). The Inter mode performs motion-compensatedprediction with transmitted Motion Vector Differences (MVDs) that can beused together with MVPs for deriving motion vectors (MVs). The Skip andMerge modes utilize motion inference methods (MV=MVP+MVD where MVD iszero) to obtain the motion information from spatial neighboring blocks(spatial candidates) or temporal blocks (temporal candidates) located ina co-located picture. The co-located picture is the first referencepicture in list 0 or list 1, which is signaled in the slice header.

When a PU is coded in either Skip or Merge mode, no motion informationis transmitted except for the index of the selected candidate. In thecase of a Skip PU, the residual signal is also omitted. For the Intermode in HM-3.0, the Advanced Motion Vector Prediction (AMVP) scheme isused to select a motion vector predictor among an AMVP candidate setincluding two spatial MVPs and one temporal MVP. In this disclosure MVPmay refer to motion vector predictor or motion vector prediction. As forthe Merge and Skip mode in HM-3.0, the Merge scheme is used to select amotion vector predictor among a Merge candidate set containing fourspatial MVPs and one temporal MVP.

For the Inter mode, the reference picture index is explicitlytransmitted to the decoder. The MVP is then selected among the candidateset for a given reference picture index. FIG. 1 illustrates the MVPcandidate set for the Inter mode according to HM-3.0, where the MVPcandidate set includes two spatial MVPs and one temporal MVP:

1. Left predictor (the first available MV from A₀ and A₁),

2. Top predictor (the first available MV from B₀, B₁, and B_(n+1)), and

3. Temporal predictor (the first available MV from T_(BR) and T_(CTR)).

A temporal predictor is derived from a block (T_(BR) or T_(CTR)) in aco-located picture, where the co-located picture is the first referencepicture in list 0 or list 1. The block associated with the temporal MVPmay have two MVs: one MV from list 0 and one MV from list 1. Thetemporal MVP is derived from the MV from list 0 or list 1 according tothe following rule:

1. The MV that crosses the current picture is chosen first, and

2. If both MVs cross the current picture or both do not cross, the MVwith the same reference list as the current list will be chosen.

In HM-3.0, if a particular block is encoded in the Merge mode, an MVPindex is signaled to indicate which MVP among the MVP candidate set isused for this block to be merged. To follow the essence of motioninformation sharing, each merged PU reuses the MV, prediction direction,and reference picture index of the selected candidate. It is noted thatif the selected MVP is a temporal MVP, the reference picture index isalways set to the first reference picture. FIG. 2 illustrates the MVPcandidate set for the Merge mode according to HM-3.0, where the MVPcandidate set includes four spatial MVPs and one temporal MVP:

-   -   1. Left predictor (A_(m)),    -   2. Top predictor (B_(n)),    -   3. Temporal predictor (the first available MV from T_(BR) or        T_(CTR)),    -   4. Above-right predictor (B₀), and    -   5. Below-left predictor (A₀).

In HM-3.0, a process is utilized in both Inter and Merge modes to avoidan empty candidate set. The process adds a candidate with a zero MV tothe candidate set when no candidate can be inferred in the Inter, Skipor Merge mode.

Based on the rate-distortion optimization (RDO) decision, the encoderselects one final MVP for Inter, Skip, or Merge modes from the given MVPlist and transmits the index of the selected MVP to the decoder afterremoving redundant MVPs in the list. However, because the temporal MVPis included in the MVP list, any transmission error may cause parsingerrors at the decoder side and the error may propagate. When an MV of aprevious picture is decoded incorrectly, a mismatch between the MVP listat the encoder side and the MVP list at the decoder side may occur.Therefore, subsequent MV decoding may also be impacted and the conditionmay persist for multiple subsequent pictures.

In HM-4.0, in order to solve the parsing problem related to Merge/AMVPin HM-3.0, fixed MVP list size is used to decouple MVP list constructionand MVP index parsing. Furthermore, in order to compensate the codingperformance loss caused by the fixed MVP list size, additional MVPs areassigned to the empty positions in the MVP list. In this process, Mergeindex is coded using truncated unary codes of fixed length equal to 5 orless, and AMVP index is coded using fixed length equal to 2 or less.

Another change in HM-4.0 is the unification of MVP positions. Both Mergeand Skip use the same positions shown in FIG. 3. For Merge mode inHM-4.0, up to four spatial MVPs are derived from A₀, A₁, B₀, and B₁, andone temporal MVP is derived from T_(BR) or T_(CTR). For the temporalMVP, T_(BR) is used first. If T_(BR) is not available, T_(CTR) is usedinstead. If any of the four spatial MVPs is not available, the blockposition B₂ is then used to derive MVP as a replacement. After thederivation process of the four spatial MVPs and one temporal MVP, theprocess of removing redundant MVPs is applied. If the number ofavailable MVPs is smaller than five after redundant MVP removal, threetypes of additional MVPs are derived and are added to the MVP list.

In the derivation for the spatial and temporal MVPs, the MVP can bederived with the MV pointing to the same reference picture as the targetreference picture. Alternatively, the MVP can be derived from acandidate MV pointing to a different reference picture. FIG. 4illustrates an example of deriving a spatial MVP based on various typesof motion vectors associated with spatial neighboring candidate blocks,where the candidate blocks comprises spatial neighboring blocks A₀, A₁,B₀, B₁ and B₂, and temporal co-located blocks T_(BR) or T_(CTR). Thecircled numbers refer to the search order for determining an MVP fromrespective candidates. The highest priority of search corresponds to anMV pointing to the target reference picture within the given referencelist. The next highest priority of search corresponds to an MV pointingto the target reference picture within the other reference list. Thethird and fourth search priorities correspond to other reference picturewithin the given and other reference lists respectively. In theparticular example of FIG. 4, the availability of motion vectors 1 and 2is checked together, and the availability of motion vectors 3 and 4 ischecked together. The availability of motion vectors 1 and 2 is checkedfrom candidate blocks A₀ through A₁ and then from B₀ through B₂. If noneof the MVs exists, the search checks the availability of motion vector 3and 4 through all the blocks. When the MVP is derived from an MVpointing to a different reference picture or the MV is for a co-locatedpicture, the MV may have to be scaled to take into consideration ofdifferent picture distances. The exemplary search patterns for MVPderivation as shown in FIG. 4 shall not be construed as limitations tothe present invention as described in this application. For example, theavailability of motion vectors 1 through 4 for each block can be checkedtogether. In another example, motion vector 1 can be checked first inthe order from A₀ to A₁ and then from B₀ to B₂. If none of the MVsexists, the search moves to check the availability of motion vector 2from A₀ to A₁ and then from B₀ to B₂. The process will continue formotion vector 3 and motion vector 4 if the spatial MVP has not beenderived.

In the derivation process for the spatial and temporal MVPs, thedivision operation is required to scale the motion vector. The scalingfactor is calculated based on the picture distance ratio. For example,the MVP may be derived based on the MV of a co-located block. Thepicture distance scaling factor, DistScaleFactor is computed accordingto:

$\begin{matrix}{{{DistScaleFactor} = \frac{{POC}_{curr} - {POC}_{ref}}{{POC}_{temp} - {POC}_{temp\_ ref}}},} & (1)\end{matrix}$where POC_(curr) and POC_(ref) represent the picture order counts (POCs)of the current picture and the POC of the target reference picturerespectively, and POC_(temp) and POC_(temp) _(_) _(ref) represent thePOC of the co-located picture and the POC of the reference picturepointed to by the MV of the co-located block respectively. While the MVof a co-located block is used to illustrate the derivation of picturedistance scaling factor, the MV of a spatial neighboring block may alsobe used to derive the MVP and the corresponding derivation of picturedistance scaling factor can be shown similarly.

In an implementation according to HM-4.0, the POC distance betweencurrent picture and the target reference picture and the POC distancebetween the co-located picture and the reference picture pointed to bythe MV of the co-located block are first constrained within a givenrange, i.e.:DiffPOC_(curr)=clip(−128,127,POC_(curr)−POC_(ref)),DiffPOC_(temp)=clip(−128,127,POC_(temp)−POC_(temp) _(_) _(ref)).

The scaling factor is then calculated according to the followingequations:

$\begin{matrix}{\mspace{79mu}{{X = \frac{2^{14} + {\frac{{DiffPOC}_{temp}}{2}}}{{DiffPOC}_{temp}}},{and}}} & (2) \\{{DistScaleFactor} = {{clip}\left( {{- 1024},1023,{\left( {{{DiffPOC}_{curr} \times X} + 32} \right)\operatorname{>>}6}} \right)}} & (3)\end{matrix}$

The scaled MVP is derived by multiplying the MV by the distance scalingfactor, i.e.:ScaledMV=sign(DistScaleFactor×MV)×((|DistScaleFactor×MV+127)>>8)  (4)

The picture scaling factor in the form of equation (1) will requiredivision operation, which is more complicated for hardware implement orconsumes more CPU time in software based implementation. The essence ofthe implementation based on HM4.0 is to pre-multiply the distance ratioby a multiplication factor (2¹⁴ in this equation (2)) so that thedistance ratio becomes an integer. In equation (2), an offset term,|DiffPOC_(temp)/2| is added to 2¹⁴ to take care of data conversion withrounding. Similarly, offsets 32 and 127 are added in equations (2) and(3) for data conversion. The multiplication factor can be compensated bysimple right-shifting operation. Therefore, the implementationassociated with equations (1) through (4) is a preferred approach sincethe computation of the scaled MV, ScaledMV does not require any divisionoperation.

In HM-4.0, the scaling factor (DistScaleFactor) is clipped to the range[−1024, 1023], as shown in equation (3). The scaling factor will beright shifted by eight bits as shown in equation (4), which implies thatthe effective scaling range is limited to [−4, 4). A reference pictureselection method for a low-delay coding system is disclosed by Li et al.to improve coding efficiency (“Encoding optimization to improve codingefficiency for low delay cases”, by Li et al., Joint Collaborative Teamon Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG116th Meeting: Torino, IT, 14-22 Jul. 2011, Document: JCTVC-F701).According to the method disclosed by Li, et al., one nearest picture andthree high quality pictures (pictures with low QPs) are used asreference pictures for low delay cases. FIG. 5 illustrates an exemplaryreference frame configuration for the low-delay system, Picture 10 isthe current picture and picture 9 is the nearest picture. Pictures 0, 4and 8 are the three high quality reference pictures. Block 525corresponds to a current block and block 515 corresponds to aneighboring block. The neighboring block 515 has an associated MV 510that is used to derive the MVP for the current MV 520 of the currentblock 525. The picture distance associated with the current MV is 6while the picture distance associated with the candidate MV is 1.Therefore, the picture scaling factor for this example is 6, whichexceeds the supported scaling factor range. Therefore, the effectivescaling range [−4, 4) is not sufficient for some applications.

Accordingly, it is desirable to develop a scheme to increase theeffective scaling ratio for MV scaling. A system incorporating theincreased scaling factor may achieve improved performance.

SUMMARY

A method and apparatus for deriving a scaled MV (motion vector) for acurrent block based on a candidate MV associated with a candidate blockare disclosed. Embodiments according to the present invention increaseeffective scaling factor of motion vector scaling. In one embodiment ofthe present invention, a distance ratio of the first picture distancebetween a current picture and a target reference picture pointed to by acurrent motion vector of the current block to the second picturedistance between a candidate picture corresponding to the candidateblock and a candidate reference picture pointed to by the candidate MVis computed. The scaled MV is then generated based on the candidate MVaccording to the distance ratio, where the scaled MV has an effectivescaling ratio between −m and n, and wherein m and n are positiveintegers greater than 4. The values of m and n can be 8, 16 or 32. Inanother embodiment, the distance ratio is related to a first scalingfactor and a second scaling factor, the first scaling factor is relatedto the first distance value and the second scaling factor is related tothe second distance value. The first scaling factor is then generated byclipping a product value to a range between −p and (p−1), where theproduct value is related to a first component derived by multiplying thefirst distance value by the second scaling factor and shifting the firstcomponent to the right by 8 bits, and where p is larger than 1024. Thescaled MV is related to a second component derived by multiplying thecandidate MV by the first scaling factor and shifting the secondcomponent to the right by 6 bits. The value of p can be 2048, 4096 or8192.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of neighboring block configuration used toderive MVP candidate set for Inter mode based on Advanced Motion VectorPrediction (AMVP) scheme in HM3.0.

FIG. 2 illustrates an example of neighboring block configuration used toderive MVP candidate set for Skip and Merge modes in HM3.0.

FIG. 3 illustrates an example of neighboring block configuration used toderive MVP candidate set for AMVP/Merge modes in HM4.0.

FIG. 4 illustrates an exemplary search order of MVP list for AdvancedMotion Vector Prediction (AMVP) in HM4.0.

FIG. 5 illustrates an example of reference frame configuration for a lowdelay coding system.

FIG. 6 illustrates an exemplary flow chart of a system incorporating anembodiment of the present invention, where the effective scaling ratiois increased beyond the range between −4 and 4.

DETAILED DESCRIPTION

As mentioned before, the scaling factor range is insufficient for somevideo coding systems, such as a low delay system using one nearestpicture and three highest quality pictures. To overcome the insufficientscaling factor range issue associated with MVP derivation, embodimentsof the present invention increase the effective scaling range. A systemincorporating an embodiment of the present invention may have aneffective scaling range [−8, 8), [−16, 16), or [−32, 32) to accommodatethe reference pictures with longer temporal distances.

In the motion vector predictor (MVP) derivation according to HM-4.0, theMV is scaled to the target reference picture as the final MVP when anMVP is derived from an MV pointing to a different reference picture. Inthe MV scaling process, the scaling factor is defined by equation (5):ScalingFactor=(POC_(curr)−PoC_(ref))/(POC_(col)−POC_(col) _(_)_(ref))=tb/td,  (5)where td is the POC distance between the co-located picture and thereference picture pointed to by the MV of the co-located block, and tbis the POC distance between the current picture and the target referencepicture. The scaling factor for MVP derivation based on a spatialneighboring block can be computed similarly. In HM-4.0, the scalingfactor is calculated according to:X=(2¹⁴ +|td/2|)/td, and  (6)ScalingFactor=clip (−1024,1023,(tb×X+32)>>6).  (7)Then, the scaled MV is derived as follows:ScaledMV=sign(ScalingFactor×MV)×((abs(ScalingFactor×MV)+127))>>8)  (8)

Embodiments according to the present invention increase the effectivescaling ratio by increasing the clip value of equation (7) to 2048,4096, or 8192. For example, equation (7) can be changed as follow:ScalingFactor=clip (−2048,2047,(tb×X+32)>>6),  (9)ScalingFactor=clip (−4096,4095,(tb×X+32)>>6), or  (10)ScalingFactor=clip (−8192,8191,(tb×X+32)>>6).  (11)

Accordingly, the effective scaling range can be increased to [−8, 8),[−16, 16), or [−32, 32) respectively.

The division free operation associated with picture distance scaling isaccomplished via a value X formed by multiplying (l/td) by 2¹⁴ as shownequation (6). The multiplication factor 2¹⁴ is later compensated by8-bit right shift during derivation of ScalingFactor as shown inequation (7) and 6-bit right shift during derivation of scaled MV asshown in equation (8). While shifting by 8 bits and 6 bits in equations(7) and (8) respectively are used, other configurations may also beused. For example, instead of right shifting by 8 bits and 6 bits, itcan also be configured to right shift by 7 bits and 7 bits in equations(7) and (8) respectively. In this case, since ScalingFactor is derivedbased on (tb×X) by right shifting 7 bits instead of 6 bits, theresulting value for ScalingFactor is scaled down by a factor of 2.Accordingly, the clip range should also be scaled down by a factor of 2.In this case, the clip values in equations (9) through (11) will become1024, 2048 and 4096 respectively. Accordingly, equation (6) can begeneralized as:ix=(2^(k) +|td/2|)/td, and  (12)where k is an integer and k=q+m. Both q and m are integers, where qcorresponds to the right shift during derivation of distance scalingfactor, DistScalefactor and m corresponds to the right shift duringderivation of the scaled MV, scaled_MV_xy. Both DistScalefactor andscaled_MV_xy can be derived as:DistScalefactor=clip(−p,(p−1),(tb*ix+(1<<(q−1)))>>q), and  (13)scaled_MV_xy=(DistScaleFactor*MV_xy+(1<<(m−1))−1+(((DistScaleFactor*MV_xy)<0)?1:0))>>m  (14)where p is an integer associated with the desired clipping range and pis larger than 1024. For example, p may correspond to 2048, 4096 or 8192and q and m may correspond to 6 and 8 respectively.

FIG. 6 illustrates an exemplary flow chart for a system incorporating anembodiment of the present invention. The first picture distance betweena current picture corresponding to the current block and a targetreference picture pointed to by a current motion vector of the currentblock is determined in step 610. The second picture distance between acandidate picture corresponding to the candidate block and a candidatereference picture pointed to by the candidate MV of the candidate blockis determined in step 620. The distance ratio corresponding to a ratioof a first distance value to a second distance value is determined instep 630. The first distance value is associated with the first picturedistance and the second distance value is associated with the secondpicture distance. The scaled MV is then generated based on the candidateMV according to the distance ratio in step 640, wherein the scaled MVhas an effective scaling ratio between −m and n, and wherein m and n arepositive integers greater than 4. In some embodiments, the values of mand n can be 8, 16 or 32. The flow chart in FIG. 6 is intended toillustrate an example of motion vector scaling with increased effectivescaling ratio. A skilled person in the art may practice the presentinvention by re-arranging the steps, split one or more steps, orcombining one or more steps.

The distance ratio in step 630 can be calculated by a first scalingfactor and a second scaling factor, where the first scaling factor isrelated to the first distance value determined in step 610, and thesecond scaling factor is related to the second distance value determinedin step 620. The first scaling factor is then generated by clipping aproduct value to a range between −p and (p−1), where the product valueis related to a first component derived by multiplying the firstdistance value by the second scaling factor and shifting the firstcomponent to the right by 8 bits, and where p is larger than 1024. Thescaled MV is related to a second component derived by multiplying thecandidate MV by the first scaling factor and shifting the secondcomponent to the right by 6 bits. The value of p can be 2048, 4096 or8192.

The above description is presented to enable a person of ordinary skillin the art to practice the present invention as provided in the contextof a particular application and its requirement. Various modificationsto the described embodiments will be apparent to those with skill in theart, and the general principles defined herein may be applied to otherembodiments. Therefore, the present invention is not intended to belimited to the particular embodiments shown and described, but is to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed. In the above detailed description, variousspecific details are illustrated in order to provide a thoroughunderstanding of the present invention. Nevertheless, it will beunderstood by those skilled in the art that the present invention may bepracticed.

Embodiment of the present invention as described above may beimplemented in various hardware, software codes, or a combination ofboth. For example, an embodiment of the present invention can be acircuit integrated into a video compression chip or program codeintegrated into video compression software to perform the processingdescribed herein. An embodiment of the present invention may also beprogram code to be executed on a Digital Signal Processor (DSP) toperform the processing described herein. The invention may also involvea number of functions to be performed by a computer processor, a digitalsignal processor, a microprocessor, or field programmable gate array(FPGA). These processors can be configured to perform particular tasksaccording to the invention, by executing machine-readable software codeor firmware code that defines the particular methods embodied by theinvention. The software code or firmware code may be developed indifferent programming languages and different formats or styles. Thesoftware code may also be compiled for different target platforms.However, different code formats, styles and languages of software codesand other means of configuring code to perform the tasks in accordancewith the invention will not depart from the spirit and scope of theinvention.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described examples areto be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

The invention claimed is:
 1. A method of deriving a scaled MV (motionvector) for a current block based on a candidate MV associated with acandidate block to provide an effective scaling ratio greater than 4,the method comprising: determining first picture distance between acurrent picture corresponding to the current block and a targetreference picture pointed to by a current motion vector of the currentblock; determining second picture distance between a candidate picturecorresponding to the candidate block and a candidate reference picturepointed to by the candidate MV of the candidate block; determining apre-scaled distance division having a first value related to dividing apre-scaling factor by the second picture distance, wherein thepre-scaling factor corresponds to 2^(k), k=m+q and k, m and q arepositive integers; determining a scaling factor by multiplying the firstpicture distance by the pre-scaled distance division, right-shiftingfirst multiplication result by q bits, and clipping first right-shiftingresult to a range from −p to (p−1) to obtain a final scaling factor; andgenerating the scaled MV by multiplying the candidate MV by the finalscaling factor, and right-shifting magnitude of second multiplicationresult by m bits, encoding or decoding the current block based on thescaled MV with the effective scaling ratio; wherein k, m and q are setto 14, 8 and 6 respectively and p is set to 2^((m+3)), 2^((m+4)) or2^((m+5)) to cause the effective scaling ratio between the scaled MV andthe candidate MV to be 8, 16 and 32 respectively.
 2. The method of claim1, wherein the candidate block corresponds to a spatial neighboringblock or a temporal co-located block.
 3. A method of deriving a scaledMV (motion vector) for a current block based on a candidate MVassociated with a candidate block to provide an effective scaling ratiogreater than 4, the method comprising: determining first picturedistance, tb between a current picture corresponding to the currentblock and a target reference picture pointed to by a current motionvector of the current block; determining second picture distance, tdbetween a candidate picture corresponding to the candidate block and acandidate reference picture pointed to by the candidate MV of thecandidate block; determining a pre-scaled distance division, ixaccording to ix=(2^(k)+|td/2|)/td, wherein k=m+q and k, m and q arepositive integers; determining a scaling factor, DistScalefactoraccording to DistScalefactor=clip(−p, (p−1), (tb*ix+(1<<(q−1)))>>q),wherein clip(u,v,w) is a clipping function that limits w to a rangebetween u and v; generating a scaled MV, scaled_MV_xy based on acandidate MV, MV_xy according toscaled_MV_xy=(DistScaleFactor*MV_xy+(1<<(m−1))−1+(((DistScaleFactor*MV_xy)<0)?1:0))>>m;and encoding or decoding the current block based on the scaled MV withthe effective scaling ratio; wherein k, m and q are set to 14, 8 and 6respectively and p is set to 2^((m+3)), 2^((m+4)) or 2^((m+5)) to causethe effective scaling ratio between the scaled MV and the candidate MVto be 8, 16 and 32 respectively.
 4. The method of claim 3, wherein thecandidate block corresponds to a spatial neighboring block or a temporalco-located block.